The present invention relates generally to the field of electronic circuits and, in particular, to a variable attenuator.
Coaxial cable networks are a common medium that is used to distribute video and audio programming to consumers with a higher quality than is typically achieved using conventional antennas connected to a television at each user""s location. A cable network typically includes a head end that receives inputs or programming from a number of content providers, e.g., ABC, NBC, CBS, Fox, CNN, ESPN, etc. The head end is typically connected to a distribution network that distributes the programming from the head end to, for example, the television sets of a number of subscribers. The distribution network can include coaxial cable alone or in combination with optical fiber, or other distribution medium.
Radio frequency (RF) signals transmitted over a coaxial cable portion of the distribution network tend to attenuate as a function of distance from the head end. This means that the RF signals decrease in quality as they get further away from the head end. To compensate for this attenuation, cable networks typically include network amplifiers that are selectively distributed throughout the network. These amplifiers receive and amplify the RF signals at these selected points in the network such that the signals provided to each end user provide an acceptable level of quality.
A typical network amplifier is formed on a circuit board that is disposed within a housing. The circuit board includes a number of interconnected circuit components that are coupled together to provide, among other things, an RF signal path. The RF signal path is used to controllably amplify RF signals transmitted along the path.
At points in the network amplifier, it may be necessary to reduce the magnitude of the RF signal so that the amplifier amplifies the signal within specified parameters. This is conventionally accomplished through the use of at least one attenuator pad. Attenuator pads are prefabricated devices that plug into a specified location on a circuit board within the amplifier housing. Typically, each attenuator pad provides a preselected, fixed attenuation level. Thus, the amplifier manufacturer determines the desired attenuation level and plugs in an attenuator pad that meets the desired attenuation level.
Once the network amplifier is adjusted, including placement of appropriate attenuator pads, the housing of the network amplifier is sealed and the amplifier is placed into a network. With the amplifier in place, it may become necessary to adjust the attenuator pad. To do this, a technician must go the location of the network amplifier, open the housing and physically remove the original attenuator pad and insert a replacement attenuator pad with an appropriate attenuation value.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an attenuator that can more easily be adjusted.
The above-mentioned problems with attenuators and other problems are addressed by the present invention and will be understood by reading and studying the following specification. A variable attenuator is described which provides a selected attenuation based on a value of at least one digital control signal.
In particular, an illustrative embodiment of the present invention includes a variable attenuator formed from a combination of PIN diodes. The PIN diodes may be coupled in a xe2x80x9cpi,xe2x80x9d xe2x80x9cTxe2x80x9d or other appropriate configuration. At radio frequencies (RF), a PIN diode acts as a variable resistor with a resistance value based on the bias current of the PIN diode. To control the attenuation level of the variable attenuator, the bias current of the PIN diodes are selectively adjusted. Digital values relating to selected bias currents, and thus selected attenuation levels, are stored in a memory. These digital values are provided as control signals that set the bias current levels for the PIN diodes. The bias current levels control the attenuation level of the variable attenuator.
In another embodiment, the variable attenuator is provided in series with a fixed attenuator. The fixed attenuator can be switchedly bypassed. If the fixed attenuator provides the same attenuation level as the maximum level for the variable attenuator, the combination of the fixed and variable attenuators can provide up to twice as much attenuation as the variable attenuator working alone.